Polyethylene is the most widely used commercial polymer and can be prepared by different processes. For instance, polymerization in the presence of free-radical initiators at elevated pressures was the method first discovered for producing polyethylene and continues to be a valued process with high commercial relevance for the preparation of low density polyethylene (LDPE).
A normal set-up of a plant for preparing low density polyethylene comprises a polymerization reactor, which can be an autoclave or a tubular reactor or a combination of such reactors, and further equipment. For pressurizing the reaction components, usually a set of two compressors, a primary and a secondary compressor, is used. At the end of the polymerization sequence, a high-pressure polymerization unit may further include apparatuses like extruders and granulators for pelletizing the resulting polymer. Furthermore, the polymerization unit may also comprise means for feeding monomers and comonomers, free-radical initiators, modifiers or other substances at one or more positions to the polymerization reaction.
A characteristic of the radically initiated polymerization of ethylenically unsaturated monomers under high pressure is that the conversion of the monomers is generally not completed after one round (pass) of processing. For instance, for each pass of the reactor, only about 10% to 50% of the dosed monomers are converted in a polymerization process performed in a tubular reactor, and from 8% to 30% of the dosed monomers are converted in the case of a polymerization in an autoclave reactor. Accordingly, it is common practice to separate the discharged reaction mixture into polymeric and gaseous components and recycle the monomers. To avoid unnecessary decompression and compression steps, the separation into polymeric and gaseous components is usually carried out in two stages. The monomer-polymer mixture leaving the reactor is transferred to a first separating vessel, frequently called the high-pressure product separator, in which the separation in polymeric and gaseous components is carried out at a pressure that allows for recycling of the ethylene and comonomers separated from the monomer-polymer mixture to the reaction mixture at a position between the primary compressor and the secondary compressor. At the conditions of operating the first separation vessel, the polymeric components within the separating vessel are in liquid state. The level of the liquid phase in the first separating vessel is generally measured by radiometric level measurement and is controlled automatically by a product discharge valve. The liquid phase obtained in the first separating vessel is transferred to a second separation vessel, frequently called the low-pressure product separator, in which further separation in polymeric and gaseous components takes place at lower pressure. The ethylene and comonomers separated from the mixture in the second separation vessel are fed to the primary compressor where they are compressed to the pressure of the fresh ethylene feed, combined with the fresh ethylene feed, and the joined streams are further pressurized to the match the pressure of the high-pressure gas recycle stream.
The first separation vessel, which generally operates at a pressure in the range of from 10 MPa to 50 MPa, is usually equipped with safety devices for protecting the vessel from over-pressurization. The commonly utilized devices are bursting discs, which are usually installed at the exit lines of the separation vessels through which the gaseous fraction exits the separation vessel for recycling to the secondary compressor. To prevent blocking of the bursting discs by polymer entrained by the recycle gas, dead space in front of the bursting discs should be avoided. For safety reasons, the bursting discs may be installed within a massive steel block.
Modern world-scale plants are generally designed with a higher capacity for a single production line than older plants. Due to the higher throughput, separating vessels of a larger dimension may be used, and accordingly the volume which has to be depressurized by failing bursting discs increases. Since the diameter of available bursting discs is limited, this set-up requires the installation of more than one bursting disc for sufficiently fast pressure release. Moreover, the installation of a bursting disc unit requires a certain assembling volume and the bursting discs should be installed directly at lines with permanent gas flow to avoid “dead space.” It has therefore become necessary to convey the gas exiting the separation vessel through more than one exit line to establish enough capability for installing bursting discs in the proximity of the first separation vessel.
Accordingly, it was the objective of the present disclosure to overcome the disadvantages of the prior art and provide a separation vessel with a dead-space-free installation of one or more busting discs, where separation vessel has a relatively simple design and can be constructed economically, where the separation vessel does not require a split of the gas leaving the separation vessel to pass through more than one exit line, and an installation of bursting discs in separate massive steel blocks can be advantageously avoided.